Principles of Centrifugal Rubber Mold Casting : Chapter 4

1. Introduction to pewter alloys
2. How modern pewter alloys are manufactured
3. Pewter alloy contamination: secondary metals and carelessness in the shop
4. How pewter casting alloys behave when they are melted
5. Why pewter casting alloys behave the way they do
6. Metals used in pewter alloys
7. Pewter alloy weight and how to substitute one alloy for another
8. Dross: what it is, where it comes from, what it tells you
9. Fluxing, what it does and how to do it
10. Pewter alloy cost
11. Pewter scrap and drosses
12. Notes on pewter inventory control
13. Other casting materials; fusible alloys and plastics
14. A note on the re-use of gates

4.1: Introduction to pewter alloys

In addition to designing the mold so that it will cast properly, successful casting depends on selecting the right pewter alloy for each job. However, it is not always easy to predict which alloy will perform best in a given mold. High tin alloys, while generally excellent for casting, do not always perform as well as less expensive, lower tin content alloys, especially when casting heavy boutique castings. Further, a mold that casts beautifully using one supplier’s alloy may not always perform as well when cast with a similar alloy from a different supplier. Because pewter alloys affect casting in so many ways, practical knowledge of metals and alloys is essential to the moldmaker, the caster, and the management of the modern casting shop.

An alloy is made by combining two or more different metals for the purpose of improving one metal, changing some of its characteristics such as weight or melting temperature, or producing a metal that is lower in cost.

Good alloying always begins with virgin metals. These come directly from the primary smelter. When choosing metals for alloying, the alloyer must also pay careful attention to purity and grade, since there are many grades of virgin tin, lead, and antimony, some of which may be suitable only for some alloys but not others.

For example, virgin Grade B tin is not the same as, nor will it perform as well as, Grade A Straits tin. Only with Grade A Straits have traces of lead been controlled enough to meet the strict requirements of the American Pewter Guild, Ltd. for making Oster’s Certified American Pewter. (TM)

4.2: How modern pewter alloys are manufactured

Manufacturing an alloy that can always be relied on to perform according to specifications involves more than simply melting together the constituent metals in approximately correct proportions in a melting furnace. A reputable pewter alloyer such as the A. J. Oster Company goes through elaborate manufacturing procedures in making pewter alloys to ensure that they will perform consistently each and every time they are purchased.

An alloy of a given composition purchased today should perform precisely as metal of the same type did a month or a year ago. To guarantee this consistent performance, it is essential that the precise parameters of weight, temperature, time, and relative proportions all be scrupulously controlled by a well equipped in-plant laboratory. (Fig. 4.1) For this reason, manufacturing pewter alloys is beyond the technical capacity of most casting shops, re-melters, and scrapyards.

Fig 4.1: A modern analytical laboratory is essential to the manufacture of quality casting alloys (partial view of the Oster laboratory)

To prepare our pewter alloy so that it will have the characteristics predicted by its specifications, virgin metals of the specified grade and purity are weighed on electronic scales to a tolerance of ±0.2%, for each standard 1800 pound melt (or ‘heat’). Temperature is maintained within ±10°F.

Each metal used in the alloy must be entered into the melting pot at a specific temperature. For example, when lead is added to tin, the heat must be lowered before the lead can be entered to prevent the tin from drossing out of the melt immediately. Not until it is completely molten can another metal be entered. Antimony which melts at 1166°F and copper which melts at 1982°F can be entered into an alloy of tin or lead at 600°F, but it requires special techniques to do so successfully. (See Chapter 4.5)

As metals are entered into an alloy, drosses appear. Proper fluxing techniques and special fluxes must be used to ensure that the alloy remains homogeneous and that its proportions do not change because metal is lost in the drosses.

It is important to stir the molten metal during alloy preparation to ensure that the metals enter the melt completely. The finished alloy must be poured at a specific temperature and time to ensure that each ingot of alloy is homogeneous and has the same composition. If it is poured too cold, the ingots will have a pasty appearance; whereas, if it is too hot, the alloy may lose some of its tin content. When ingots are poured, they must solidify quickly to ensure homogeneity and a fine crystal structure. (Fig. 4.2) Slow solidification can cause the formation of large, coarse crystals which may cause problems when the alloy is remelted for casting. Each pewter ingot must be skimmed after pouring to remove all traces of dross. Pewter ingots that have not been skimmed will not have a shiny, mirror-like finish. (Fig. 4.3)

During alloy manufacturing continuous laboratory supervision is necessary to ensure that the alloy conforms to specifications before it is poured into the ingot mold. Since neither in ingot form nor in their molten state do alloys reveal their weaknesses and shortcomings, the alloyer’s reputation is the only reliable guide to selecting a supplier whose products can be trusted.

But, it is also the caster’s responsibility to maintain the quality of the alloy he uses. To assume that all that is required of the caster is that he simply put the ingots in the pot and melt them is inconsistent with proper metal maintenance procedures. The caster must understand and follow vigorously procedures for maintaining molten pewter metal. Any carelessness will increase the speed of metal depreciation, which, in turn, will raise costs, reduce casting quality, and depress production drastically. (Chapter 2.2: Caster and the pot)

4.3: Pewter alloy contamination: secondary metals and carelessness in the shop

Absolutely pure alloys are crucial to successful casting. For this reason, conscientious casters use only alloys made from 100% virgin metals. Alloys made from secondary metals, recycled metals, or scrap metals that have been used previously in such applications as batteries, pipes, solders, and jewelry simply cannot perform predictably because they carry trace elements from their previous uses that give them unpredictable and undesirable characteristics. Lead, for example, after repeated melting, can become hard and brittle because of oxide formations which may, in turn, lead to major casting problems. Tin content will be reduced with repeated usage.

Secondary metal alloyers use metals that come from many different and usually unknown sources. Their pewter arrives in pigs, junked castings, and drosses in lots of 500 pounds or more. With each lot comes an analysis of approximate content given as percentages of tin, lead, and antimony. The secondary alloyer adds to this scrap the quantities of different metals necessary to bring the melt to the proportions required for the alloy he wants to make.

Fig 4.2: Pouring Ostercast pewter alloy to ingot form

However, because the secondary alloyer does not receive his scrap from a single, constant source, the analyses he receives are not always reliable. Furthermore, no analysis of scrap ever includes its ‘tramp metals’ content, the trace percentages of metals such as zinc, iron, aluminum, and copper, since the cost of analyzing all of the metals present only in traces in a lot is prohibitive. Thus the alloys that the secondary alloyer makes and sells cannot perform consistently from melt to melt because their contents will vary widely. (Especially the melts you buy from him!) The tramp elements in these secondary alloys may be ‘freebees’, but they are also contaminants that can cost the purchaser a lot in time, money, and quality.

Fig 4.3: Skimming ‘just cast’ pewter ingots of Ostercast pewter alloy helps remove any residues of dross, ensuring a cleaner alloy for the customer.

Some of their effects are:

Zinc: Alloys contaminated with zinc will be sluggish and lack fluidity. They will require higher casting temperatures and more fluxing. Roughness, porosity, poor reproduction of details. hard spots, and a grainy or frosted appearance in the finished casting are just a few of the problems. Zn is a dross former.

Nickel: Nickel contamination causes blistering on the casting surface.

Arsenic: The effects produced by arsenic are similar to nickel. Arsenic comes from battery leads.

Aluminum: Aluminum contamination will produce the same effects as zinc. The source is pop-tops from aluminum soda cans. Al is a dross former.

Calcium: Calcium produces heavy drosses in a melt. The lead from the new maintenance-free batteries is the main source of calcium.

Copper: Copper will increase the hardness of an alloy and raise its working temperature. The source is babbitt metals.

Iron: The effects that iron produces are similar to those of zinc contamination. A crucible that has not been treated with “pot black” is sometimes the source of iron contamination. Fe causes hard spots (i.e. FeSn).

In pewter ingot form, poor quality alloys are difficult to distinguish from high quality metals. Even when they are molten, the difference is not always apparent. However, some indicators of probable alloy contamination are easy to recognize once the alloy is in the pot, if the caster knows what to look for. They include:

1. Heavy drosses.
2. Higher working temperatures needed for the alloy contrary to the specifications called for by the manufacturer.
3. Low metal fluidity.
4. Necessity for constant fluxing.
5. Poor quality castings.
6. Porosity in castings.
7. Shortened mold life.
8. Lack of detail in castings.
9. Round edges on castings where those in the mold cavity are sharp and clear.

Virgin metal is always the best metal to alloy with. However, not all virgin metals make good alloys, nor are all non-virgin pewter alloys bad, but they will be inconsistent from melt to melt and unpredictable in their performance. Virgin Ostercast Pewter Alloys always perform consistently, even when the ingots come from different melts made months or years apart. The small savings that secondary alloys offer initially can be more than wiped out over the long run by the cost of the problems and lost production that they may cause.

Alloy contamination in the plant: Avoiding secondary alloys does not provide a caster with a fail-safe guarantee that he will never have trouble with contaminated metal. Many of the more serious contamination problems that occur are caused right in the casting room: a moment of negligence, a zinc finding tossed into the melting pot, and the entire pot of metal is ruined. As little as 0.001% of zinc can change completely the characteristics of the alloy in a standard 100 pound melt. Contamination can also result when an alloy is worked carelessly or incorrectly. (See Chapter 2, the caster and the pot) It is also the responsibility of the caster to be continuously on guard against alloy contamination and to maintain the integrity of the alloy he is using scrupulously.

4.4: How pewter casting alloys behave when they are melted

Pewter casting alloys are a class of metals that have been engineered to melt and solidify in a way that is quite different from the way that pure metals - and alloys that behave like pure metals - do. It is the way that casting alloys melt and solidify that gives them the capacity to fill the cavities in a rubber mold and reproduce their details faithfully. Because pure metals behave differently, they simply cannot do the job. It is the unique properties of specially formulated casting alloys that alone make CRMC possible. For this reason, the most practical knowledge anyone involved in casting can have is a thorough understanding of how casting alloys behave when they melt and solidify, and of how their behavior differs from that of pure metals.

How pewter casting alloys differ from other metals: As a pure metal such as lead or tin is heated, it melts, or changes from a solid to a liquid, when it passes a certain temperature. If the metal is then allowed to cool, it will solidify or change from a liquid back into a solid as it passes the same temperature. The temperature at which a metal melts and solidifies is the ‘melting point’ for that metal. Different metals have different melting points. The melting point of tin is 450°F; that of lead is 621°F. What is important to remember is that, under ordinary conditions, whenever a pure metal is heated above its melting point, it becomes completely liquid through and through. Whenever it is cooled below its melting point, it becomes completely solid through and through. Any pewter alloy that has a single melting point above which it is completely molten and below which it is completely solid is called a eutectic. Some alloys however, are eutectics such as linotype metal (4% tin, 11% antimony, 85% lead) which is eutectic at 463°F, solder (63% tin. 37% lead) which is eutectic at 361°F and Ostercast Alloy 12AL (12% antimony, 88% lead) which is eutectic at 486°F.

Solidus and liquidus: The pewter casting alloys used in the CRMC process are non-eutectic compositions. When they melt and solidify, they behave very differently from the eutectics. The important terms to use when describing how casting alloys melt and solidify are solidus, ‘pasty range’ and liquidus.

As a pewter casting alloy is heated, it begins to melt when it reaches a certain temperature. This temperature is termed the alloy’s solidus. But the alloy does not become completely molten through and through until it has been heated still further to a yet higher temperature. This temperature above which the alloy finally becomes completely molten is termed the alloy’s liquidus. (The difference between the solidus and liquidus of a non-eutectic alloy is termed the melting or ‘pasty range’.) When a casting alloy that is completely molten is cooled, it begins to solidify as it passes its liquidus.

Pasty range: The range of temperatures between an alloy’s solidus and its liquidus is termed the alloy’s ‘pasty range’, because a casting alloy heated to any temperature between its solidus and its liquidus will be neither completely solid nor completely liquid, but pasty. It is because casting alloys do not melt and solidify at a single, sharply defined temperature (as do eutectic metals) that they can be used to cast rubber molds. It is the pasty range that makes CRMC possible.

What the caster needs to know about alloy temperatures: Before a caster can begin to run any alloy, he needs to know three things: the alloy’s solidus, its liquidus, and the length of its pasty range.

He needs to know the liquidus because, as a general rule, in order to cast a rubber mold successfully, an alloy must be run at least 50°F above its liquidus. (The technical term for this higher temperature is ‘super heat.’) But, high temperatures cause dross to form more rapidly. To prevent the alloy from losing important constituents in dross, and, so, depreciating and changing its characteristics, it should be run at the lowest temperature over 50°F above its liquidus at which it will fill the mold cavities completely and produce acceptable castings.

Why the length of an alloy’s pasty range is important: The length of an alloy’s pasty range is the number of degrees between its solidus and its liquidus. Different alloys have pasty ranges of different lengths. (See Table 4.4, where the solidi, liquidi, and pasty ranges of a selection of Ostercast alloys are compared.) The most important thing a caster needs to know about the alloy he is working with is whether its pasty range is long, average, or short.

Knowing the length of an alloy’s pasty range is important because the amount of heat an alloy must dissipate in order to cool from liquidus to solidus determines directly how long it will take the alloy to solidify completely. How long it takes an alloy to solidify determines what kinds of castings the alloy will cast successfully, as well as the size, length, and overall design of the gating system necessary to fill the cavities of the mold in which the metal is used. In general, the shorter the pasty range, the larger the gates must be. The length of the alloy’s pasty range also determines how long it will maintain a ‘solidification shrinkage feeder head’ during casting. (See Chapter 9 for an explanation of the ‘solidification shrinkage feeder head’ and how it works.) It is the ‘solidification shrinkage feeder head’ that makes CRMC possible. Because an eutectic metal does not have a pasty range, it will not maintain a ‘solidification shrinkage feeder head’, and, so, will solidify in the gates of a rubber mold before the cavities have filled completely. This is why an eutectic metal cannot be used in CRMC.

Fig 4.4: Solidi, liquidi, and ‘pasty range’ of selected Ostercast Alloys

Running a eutectic metal or an alloy with a very short pasty range at a very high temperature in order to compensate for the lack of a ‘solidification shrinkage feeder head’ will increase the metal’s fluidity. But the high temperature will produce heavy drosses and cause premature mold burnout. This is why, when an alloy will not fill a mold because its pasty range is too short, it is far better to change to a different alloy with a longer pasty range than try to get the mold to fill by raising the pouring temperature several hundred degrees.

4.5: Why pewter casting alloys behave the way they do

In Chapter 4.4, we have described in general terms how casting alloys are different from pure metals and what a caster needs to know to work casting alloys correctly. A detailed discussion of the techniques for running a pot of molten metal and a summary outline of metal maintenance procedures are given in Chapter 2.2. In this section, we shall give a general, non-technical explanation of why alloys behave the way they do.

The nature of pewter casting alloys: The conscientious caster who observes the materials that he works with and thinks carefully about them will begin to ask himself questions:

How can most pewter casting alloys be completely molten at temperatures well below 600°F, when they contain antimony which, in its pure form, does not melt until it has been heated above 1167°F? How can antimony be alloyed with another metal without first heating the metal all the way up to the melting point of antimony?

Answers to these and other questions like them can easily be found by understanding the nature of alloys. When two metals are melted together to produce an alloy, the liquid alloy that results is not a chemical compound like water. H₂O (although in some cases, some intermetallic bonding between alloy constituents may take place).

Antimony can be alloyed with lead, tin, and other metals at temperatures far below the melting point of pure antimony because the solid antimony dissolves in the molten metal to which it is added. The result is a solution of antimony in lead or tin. The way in which one metal dissolves in another to form an alloy resembles exactly the way in which a teaspoonful of salt stirred into a glass of water dissolves in the water. In both instances, the same thing happens: the solid crystals of salt disappear in the water; the solid crystals of antimony disappear in the molten lead. In neither case do the crystals melt. Rather, they pass into solution. The result in both cases is a homogeneous fluid in which the dissolved substance is distributed equally throughout, as one can demonstrate to one’s own complete satisfaction by simply drinking the salt water. (But not the antimonial lead.)

The important point to notice about the way salt dissolves in water is this: salt will dissolve in water that is at room temperature or even cooler. Yet the melting point of salt is 1474°F! In the same way, solid antimony dissolves in molten lead that is far cooler than the melting point of antimony. The solution that results is termed an “alloy”. The antimony will remain dissolved in the lead so long as nothing happens to cause it to pass out of solution. In its solid state, an antimonial lead alloy is, technically, a ‘solid solution’.

Effect of changes in proportions of alloy constituents on alloy temperature range: Two different pewter alloys made of the same metals but in different proportions will have entirely different temperature ranges. Changing the proportions of the constituents of an alloy even slightly will alter the alloy’s characteristics and, in effect, change it into a different alloy. Thinking about alloys in terms of the analogy with a saltwater solution makes it easy to understand why changes in the proportions of alloy constituents produce this effect. (See Table 4.5, which graphs the changes in liquidus and pasty range for alloys of antimony and lead as the pro- portion of antimony in the solution is gradually increased.)

In winter, people spread salt on their sidewalks to melt the ice. They do this because, when salt is dissolved in water, it changes the water’s melting point (or, its ‘freezing point’, which is the same thing.) The more salt added to a constant volume of water, the lower the water’s freezing point becomes (that is, the freezing point drops in direct proportion to the amount of salt that is added) until, at a 23.6% concentration of salt, the solution becomes a eutectic at -5°F, 37° below the freezing point of pure water. No mixture of ice and water has a lower melting point. In the same way that salt changes the freezing point of water, dissolving antimony in lead changes the lead’s temperature characteristics. In the case of lead, adding antimony gives the lead a pasty range and a liquidus. As is shown in Fig. 4.5, the liquidus and length of pasty range of antimonial lead alloys changes in direct proportion as the percentage of antimony in the alloy is increased. Because alloys are not chemical compounds, but solutions, their temperature characteristics depend far more on the proportions of their constituents than they do on the nature of the constituents themselves. Because the changes in alloy temperatures produced by varying the proportions of the constituents can be predicted precisely, modern alloyers can “engineer” special alloys to have exactly the temperature characteristics needed for a particular job.

Why using an alloy at its recommended temperature is important: Throughout this book it has been stressed that the most important single consideration for the caster is that he work all alloys at the temperatures recommended by the alloyer. Thinking about alloys in terms of how solutions behave can help provide an understanding of why it is so important to use an alloy at a temperature as little over 50°F above its liquidus as possible. When a pewter alloy is used too hot, some of its constituents are lost in the heavy drosses that high temperatures cause. Losing even a small amount of metal in dross changes the proportions of the alloy’s constituents and transforms the remaining metal into an alloy with different characteristics.

*Fig. 4.5: Chart of temperatures for lead/antimony alloys The length of the pasty range varies for different alloys. Take for example, lead. When antimony is dissolved in lead, the physical behavior of lead changes in several important ways: (Fig. 4.5)

1. The liquidus temperature, at which the alloy commences to solidify is lower than the freezing point of lead.
2. While the first part of the alloy is solidifying the temperature continues to fall. Thus the alloy solidifies not at a single temperature but over a range of temperatures; this is called the freezing or ‘pasty’ range.
3. The final portion of the alloy solidifies while the temperature remains steady at 486°F.

As more antimony is added, the liquidus temperature is lowered still further but the alloys solidify at the same constant temperature of 486°F. Thus the freezing range becomes shorter and shorter until a composition is reached-12% antimony, 88% lead, which solidifies sharply at the single temperature of 486°F. This is the only alloy containing both antimony and lead which solidifies at one temperature. Such an alloy is known as a ‘eutectic’, which means easy melting. Ostercast makes an alloy of this composition. called ‘12AL’ which is used in bronze molds to cast trophies and lamp parts. Because it does not go through a ‘pasty range’ it solidifies quickly against the cavity walls in the bronze mold, and excess alloy is ‘slushed” out, leaving the characteristic hollow casting.

Refering to the chart Fig. 4.5, it can be seen that any addition of antimony over and above the eutectic of 12% will require the application of additional heat in order to dissolve. Thus the liquidus of the alloy, the point at which it is completely molten, becomes progressively higher, however, the solidus will remain constant at 486°F.*

Running a pewter alloy at too low a temperature can cause just as much metal loss and alloy depreciation as running it at high temperatures, although for different reasons. To understand why low temperatures cause alloys to depreciate, think again of the saltwater solution. When salt is added to a solution, a point is eventually reached at which the solution becomes “saturated”. That is, the amount of salt dissolved in the water is so great that no more will dissolve. A single crystal dropped into the glass will just sink to the bottom. But if the saturated solution is heated, it will be found that, at the higher temperature, more salt will dissolve. However, when enough salt has been added, the solution will again become saturated at this higher temperature.

As long as the saturated solution is kept at the higher temperature, the additional salt added after the temperature was raised will remain dissolved. But if the solution is allowed to cool, the excess salt will begin to separate out of the solution and form solid crystals again.

Precisely the same thing happens in a pot of molten pewter alloy being run at too low a temperature. The liquidus temperature of an alloy is really nothing more than the temperature at which the alloy is a saturated solution. Put another way, the liquidus temperature is the temperature to which the metal must be raised to get all the constituents in it to dissolve completely. (Think of the saturated saltwater solution that was heated to get more salt to dissolve in it.) An alloy of antimony and lead heated to precisely its liquidus temperature will be saturated: no more antimony will dissolve in the lead unless the temperature is raised still further. In the graph in Table 4.5, the curve of the liquidus temperatures of the antimonial lead alloys rises steadily in direct proportion to the increasing concentration of antimony in the alloy solution. The upward curve of the liquidus line represents nothing more than the increasing temperatures necessary to hold increasing concentrations of antimony in solution.

Thus when a saturated solution of antimony and lead is allowed to cool below its liquidus temperature, it behaves just as the heated saturated solution of saltwater does: some of the dissolved antimony begins to pass out of solution and form solid crystals. For this reason, any pewter alloy heated below its liquidus will contain undissolved crystals of some of its constituents. Because these crystals are lighter than the rest of the alloy, they will float to the surface of the melt.

In a casting shop, when an alloy has been run below its liquidus, the caster who does not understand alloys may mistake these solid crystals that have passed out of solution and are floating on the top of the pot for drosses, and skim them away, or, just as bad, he may dip a ladle into the alloy to cast a mold. In the one case, what he is actually doing is removing good pewter, metal that is a vital constituent of the alloy. In the other case, he is removing metal that does not have the proportions and characteristics that its specifications call for. In both cases, he changes the alloy’s proportions and destroys the characteristics that it was designed to have: the metal is no longer the alloy that the casting shop paid good money for, but a different and probably far less desirable alloy. This is why running an alloy at too low a temperature - any temperature below its liquidus - will produce alloy compositional changes every bit as severe as running it at too high a temperature. This is why an alloy, used below its liquidus, produces mis-runs, cold-shuts, castings of inferior quality, and nothing but problems.

When an alloy has been allowed to stand for any length of time at a temperature below its liquidus, the crystals that will have passed out of the alloy solution must be redissolved in the melt before the alloy can be used to cast. This is done by raising the alloy’s temperature to 50°F above its liquidus. If the metals are oxidized however, they can only be re-melted (re-alloyed) with difficulty.

What happens when an alloy melts and solidifies: To understand how metals melt and solidify, it is first necessary to have a general understanding of the structure of solid metals. All metals in their solid state are composed of crystals. Many metals such as copper, tin, and lead do not appear to be crystalline, nor do they seem crystalline because they are tough and ductile. Most people think of crystals as brittle and easily fractured. The crystals of most metals are often so small that they can be observed satisfactorily only under a microscope, and then only if the surface has been specially prepared. Sometimes exceptionally large crystals (SnSb cuboids - an alloy compound) may be visible on the surface of the metal to the naked eye.

Each pure metal has its own distinctive crystal structure. The process of solidification is the process by which a metal in its liquid or “non-crystal” state forms this crystal structure. An alloy in its solid state may consist of two, three, or more distinct kinds of crystals depending on how many different metals the alloy is made of.

The process of melting a solid metal involves heating the metal to a point just above its melting temperature, the temperature at which its crystal structure breaks down. It is not enough to raise the metal to its melting point; it must be raised above it. As any metal is heated, its atoms move more and more rapidly. When a metal melts, its atoms have been heated to a point where they are moving so rapidly that they can no longer group themselves together in crystals. Because it is the crystal structure that gives metal in its solid state its rigidity and solidity, this breakdown of organization allows the material to flow freely. In short, it changes from a solid to a liquid.

The process of solidification is the reverse of melting. As heat is taken away from a molten metal, the atoms of which it is composed move more and more slowly. When the metal reaches its melting point, the atoms are moving slowly enough that they can clump together into a rigid structure to form crystals once again. Because crystals are rigid structures, they cannot flow. The metal has become a solid.

The process of solidification is, then, for practical purposes, nothing more than the process by which a metal assumes its characteristic crystal structure. When a metal is cast, it may solidify in only a fraction of a second. In that instant the metal’s characteristic crystal structure is formed.

Because an alloy contains two or more different metals, the sequence in which its crystals form during solidification may be quite complicated. Different constituents will begin to crystallize at different temperatures. The simplest alloys are those composed of only two metals, but even these behave in a very complex way when they melt and solidify. The best way to acquire a practical idea of what happens when an alloy solidifies, then, is to consider a pure metal, bearing in mind that the constituents of an alloy do not be- have in precisely the same way, but in several different, albeit interrelated, sequences.

As an example, consider the behavior of a potful of pure lead which has been melted and is being allowed to cool. As long as it is still molten, its temperature falls steadily. But when the lead reaches its melting point at 621°F, the cooling is arrested and the temperature remains constant. Close observation of the pot at this point will reveal that there is solid metal all around the sides: what is happening is that crystals of lead are forming and growing. The temperature of all of the lead in the pot remains steady at 621°F until all of the metal has crystallized and solidified. Then and only then does the temperature begin to fall again. The heat that the metal gives off during solidification is termed the ‘latent heat of solidification’.

Other metals among these, antimony is also included are added to alloys to give them different physical characteristics in their unmelted state: ductility, strength, hardness, weight, capacity to withstand repeated bending or flexing.

Conclusion: The conscientious pewter caster who masters the foregoing simplified description of how metals behave will appreciate that the Ostercast casting alloys that he works with daily are not simply mixtures of metal that can be thrown into a pot and melted. He will realize that modern Ostercast casting alloys are sophisticated materials that must be handled with care, respect, and intelligence. They are complex metals that have been scientifically engineered to perform in precisely predictable ways. Once a caster understands the alloys that he uses, he can select an alloy for a particular casting not just on the basis of price or weight, but on the basis of how the alloy will perform in the mold’s gating system as well as on the basis of sure knowledge of which alloy will best perform in the kind of mold that he wants to cast. The knowledgeable caster can work his alloys in ways that will conserve the characteristics that Oster engineered them to have.

4.6: Metals used in pewter alloys

Because rubber molds must be cast at relatively low temperatures, only a few metals can be used in manufacturing alloys for CRMC. The two basic metals are lead and tin.

One or the other will comprise the largest proportion of any alloy used in CRMC. Varying the proportions of these metals and the amount of antimony, if used, will produce alloys that have different temperature characteristics: longer or shorter pasty ranges;

The following is a summary of the metals most often used as constituents of casting alloys and the characteristics that each gives to the alloys it is used in:

Tin (Sn) Melting point 450°F: Tin alloys well with most other metals. Its characteristics are a low melting temperature, softness, light weight as compared to lead, and high fluidity when molten. It will make lead harder without making it more brittle. Most tin alloys will produce comparatively little dross if they are properly maintained, not overheated, and made with virgin metals to begin with.

Lead (Pb) Melting point 621°F: Lead alloys well with other metals. Because of its low cost, it is economical to use. Its weight makes alloys in which it is used ideal for casting full bodied pieces, especially boutique designs and belt buckles. By itself, lead is too weak and soft for casting. It will not reproduce details with sufficient definition and clarity when cast unalloyed. When lead is added to tin it reduces the melting point of tin. For example, 5% lead will lower the solidus of the eutectic base alloy to 361°F and give it a liquidus of 432°F.

Antimony (Sb) Melting point 1166°F: Antimony makes an alloy stronger and harder and improves its capacity to reproduce detail. Antimony also changes the solidus and liquidus temperatures of the alloy to which it is added.

Copper (Cu) Melting point 1982°F: Copper hardens an alloy. 2% copper in tin will increase the tensile strength of tin 150%. Copper is used in pewter alloys only.

Cadmium (Cd) Melting point 609°F: Cadmium lowers the liquidus of an alloy. Cadmium alloys can be cast at far cooler temperatures. Cadmium also adds ductility and helps reduce shrinkage during solidification, characteristics which make cadmium alloys especially useful for casting large, flat pieces successfully. It is added to alloys in minute percentages only, in an exclusive formulation which the Oster Company has patented.

4.7: Pewter alloy weight and how to substitute one alloy for another

When selecting an alloy to use with a particular mold, it is important to know the weights of the alloys under consideration. The weight of any metal can be expressed in “pounds per cubic inch”.

If the composition of a pewter alloy is known, its weight can be found by multiplying the weight in lbs/cu.inch of each of its constituent metals by the percentage of that metal in the alloy and then adding the products together. The chart in Table 4.6 gives the weights for various Ostercast pewter alloys and for a number of pure metals in the center column. The figures in the right hand column express each metal’s weight in proportion to the weight of tin which is used as a basis of comparison.

How to substitute alloys: Sometimes it is to the caster’s advantage to cast a mold using a different alloy from that used to cast it previously. If the pewter caster has kept accurate records and knows how much of a particular alloy was needed to cast a given number of pieces in the mold, he can use the weight of the new alloy expressed as a decimal proportion of the weight of tin to figure how much of the new alloy will be needed to cast the same number of pieces. The equation is a simple proportion:

Suppose the mold required 9.9 pounds of OR8 to produce 1000 pieces when it was last cast. In order to cast 1000 pieces from the same mold using 0363 alloy, 13.38 pounds of the new alloy will be needed. This result is arrived at by setting up a proportion. ORB weighs 99 times the weight of tin. 0363 weighs 1.338 times the weight of tin. 9.9 is to x as .99 is to 1.338:

Thus it takes 13.38 pounds of 0363 to cast the same number of pieces that were cast with 9.9 pounds of OR8. This simple calculation can be extremely valuable when computing the relative costs of pewter alloys. Two alloys may possess all of the physical characteristics necessary to cast a particular mold successfully. It does not follow that the choice between the two should be in favor of the alloy with the lower unit cost. The alloy that is slightly more expensive may produce a third again as many pieces, be easier to use, and produce far fewer rejects, in which case the alloy that is more expensive per unit will be, in the long run, the more economical to use.

Fig 4.6: Weight comparisons of pure metals and selected Ostercast Pewter Alloys using tin as a base.

4.8: Dross: what it is, where it comes from, what it tells you

Dross inevitably forms on molten metal when it is exposed to air. It consists mainly of metal oxides and results from oxidation and the action of impurities in the metal. When molten metal is allowed to sit quietly in the pot without being disturbed, the rate of dross formation is quite slow because the dross that initially forms will cover the unoxidized metal, separate it from the air, and shield it from further oxidation. Any agitation or movement of the metal will increase the rate of dross formation by exposing unoxidized metal to the air. For this reason, the more careful the caster is to avoid splashing or splattering the metal when he dips (beyond what he cannot avoid when he flips the dross back to ladle clean metal), the lower the rate of dross formation will be.

The single most important cause of rapid dross formation and overly heavy drosses is working the alloy at too high a temperature. When alloys are run too hot, even constant fluxing will not prevent an exorbitant loss of good metal. Because tin oxidizes more rapidly than lead and antimony, the depreciation of tin that results from high temperatures will be especially severe, and will destroy the pewter alloy’s characteristics in a very short time.

Another cause of a high rate of dross formation, and one that is often overlooked, is air blowing across the surface of the pot. Fans, and even drafts from loose windows can increase the rate of metal depreciation. Air currents will produce dross even on a high quality alloy being run at its proper operating temperature.

Dross must not be confused with undissolved crystals of constituent metals that have floated out of the alloy because it is being run at too low a temperature. (See section 4.5) Drosses are oxides; crystals are vital metal. Crystals that have passed out of an alloy being run too cool will look very much like a heavy dross floating on the surface of the pot, but an experienced caster should be able to tell the difference. If there is any doubt at all about the identity of a substance floating on the surface of a pot of molten metal, before jumping to conclusions and fluxing and skimming the pot, check to make sure that the temperature of the metal is at least 50°F above its liquidus. If not, or if there is any reason to believe that the substance may be crystals and not dross, raise the temperature and stir the crystals back into the melt to get them to redissolve. Mistaking crystals of vital metal for dross and skimming them from the pot will cause alloy compositional changes.

How to “read” the surface of a pot of molten pewter: The appearance of dross is a good indicator of whether and how an alloy is being worked incorrectly. and also of how good the alloy was to begin with. A trained observer can “read” the surface of a pot at a glance:

Blue-red-purple coloration: Alloy is too hot. Tin is drossing out and collect- ing on the surface it may also signify the presence of lead oxides.

Lumpy appearance: Alloy is too cold. Antimony has floated out of solution.

Crystalline appearance: Alloy is too cold.

Dull surface: Even after skimming, surface remains dull instead of bright and mirror-like, or becomes dull again rapidly: the alloy is contaminated, either with zinc or aluminum.

Surface agitation: A vibrating, shivering or shimmering motion on the alloy surface indicates zinc contamination.

Keeping track of dross loss: Carefully kept records of how much dross an alloy produces can give a good indication of the alloy’s quality. High quality alloys properly made from virgin metals will always produce the least dross when they are correctly maintained and used. When dross loss is heavy for a known virgin alloy, it means that the basic procedures for running the alloy have been violated. The fault is not with the alloy, but with the way it is being used.

Many problems that are blamed on the pewter alloy, the mold, or the moldmaker are, in fact, caused by the way the caster is running the alloy or general disorganization in the casting shop. For example, in many shops it is standard procedure to skim the pots each morning as the pots are heating before the day’s work begins. The casters think they are skimming drosses. In fact, the pots have not reached the liquidus temperatures of the alloys in them, so that what these casters are really doing is skimming undissolved crystals of vital metal. Shop management then wonders why they are plagued with porosity and inferior castings.

Dross is waste! Wasted metal and wasted money! A low rate of dross formation is the reward for the shop that follows rigorously all of the procedures recommended in this book for running and maintaining a pot of molten metal.

4.9: Fluxing, what it does and how to do it

Flux is the chemical ammonium chloride (trade name ‘sal ammoniac’). Mixing a small quantity of flux into a pot of molten metal is termed ‘fluxing.’ Fluxing is an essential part of good metal maintenance. It must be done systematically and thoroughly if the alloy is to perform economically and according to specifications.

Fluxing causes the metal oxides that comprise dross to release molten metal in them so that it can re-enter the melt. After fluxing, a residue of dry, sandy slag will remain on the surface of the metal. This is skimmed off with the perforated skimmer. Fluxing, by cleaning the metal and removing dross, reduces its surface tension and so, increases its fluidity.

Skimming drosses without first fluxing the metal is extremely wasteful and causes rapid alloy depreciation. Often there are also globules of unoxidized metal entrapped in the mesh of the dross, and sometimes crystals of undissolved metal as well. Fluxing returns all of this still vital metal to the melt. Without fluxing, it is lost.

When preparing to flux a pot of molten metal, it is important not to stir the alloy or begin fluxing unless and until the metal’s temperature has been raised to 50F above its liquidus. If the metal is stirred or fluxed at too cool a temperature, undissolved crystals intermeshed in the dross will not return to the melt.

Fluxing should be done regularly: as a general rule, each time a fourth of the pewter metal in the pot has been used up, and just before gates and fresh metal are added. (See 4.14) It is not a good idea to try to keep the surface of the metal completely free of dross by fluxing too often. A thin skin of dross actually provides the unoxidized metal beneath it with a protective cover that separates it from the air and retards oxidization and dross formation. The caster can flip this skin of metal back each time he dips.

Dross in the pot only becomes a problem when there is so much of it on the metal that the caster can no longer dip without getting some dross in the ladle along with the clean metal. When this begins to happen, the metal should definitely be fluxed, otherwise dross will be poured into the molds and wind up in the castings.

Fluxing procedure:

1. Bring the pot of molten metal up to a temperature at least 50°F above the alloy’s liquidus.
2. Place the flux (ammonium chloride) in a ladle that has a handle long enough to reach to the bottom of the pot. For a standard 160 pound crucible, use one tablespoon for each quarter level of metal used.
3. Plunge the flux beneath the surface of the molten metal all the way to the bottom of the pot. Work the flux through the metal thoroughly, beginning at the bottom of the pot and working upward to the top.
4. With the ladle or skimmer, work the flux through the metal until the dross on the surface appears dry and powdery. (Fig. 4.7) Avoid splashing the molten metal.
5. Remove the dross by skimming with a perforated skimmer.” (Fig. 4.8) Shake the skimmer slightly after skimming to allow globules of molten metal to return to the pot. Place the powdery dross in a closed metal container for sale later with other scrap. An OSHA approved dust mask must be used to prevent inhalation of toxic dusts. “Preheating the skimmer and spraying it with graphite will prevent dross from adhering to it during fluxing)
6. After fluxing the surface, the surface of the molten metal in the crucible should appear bright and mirror-like.
7. After fluxing has been completed, add gates and new metal to the melt in a 50/50 ratio. (See 4.14)

4.10: Pewter alloy cost

The cost of an alloy is based on the cost of its constituent metals on the day of purchase. To the metal cost is added the alloyer’s alloying charge. Most white metals are priced at the tin price plus an alloying charge. Prices of low tin alloys are based on the percentage of tin content multiplied by the tin cost, along with the lead and antimony con- tent multiplied by their respective costs, plus an alloying charge.

Fig 4.7: Fluxing molten metal. Fig 4.8: Skimming dross after fluxing.

4.11: Pewter scrap and drosses

All scrap castings, gates, and drosses should be collected and properly stored in metal containers with covers. Cardboard boxes must never be used for hot metal skimmings. Scrap containers are not trash containers, Aluminum cans, pop tops, cigarette butts, paper, and floor sweepings must never be put in them. This must be strongly impressed upon all casting shop personnel. Protective dust masks must be used when sweeping or handling drosses because of their toxic nature.

4.12: Notes on pewter inventory control

Metal is money!

Metal represents one of the biggest expenditures for most companies that do casting. Because metal is valuable, it should be monitored as carefully as the controls a company keeps on its accounts receivables. Yet many plants fail to maintain adequate controls on this valuable commodity.

In a large casting plant, management cannot be everywhere, so authority must necessarily be delegated. Proper control must begin with a flow chart of incoming material. Is the material actually coming into the plant, or only a signed slip stating that the material has been received? Is the weight stated on the receiving slip actually the correct weight received? Is it the correct alloy at the correct price? ce? Is it virgin metal? Is the alloy, once received, haphazardly placed under a caster’s bench, thus making him responsible for perhaps $10,000 worth of metal? The following questions should be used as a guideline to provide a framework for formulating the proper metal control procedures:

1. Who has the responsibility for purchasing alloys? Management, the caster, or the sweeper?
2. How is metal checked into the plant? By weight? By alloy type? By job?
3. How is the metal inventoried and stored? Under lock or under the casting bench?
4. Are records kept so that it can be determined whether purchases minus usage equals balance on hand?
5. How is metal issued to the caster? By requisition slips or simply taken by the caster as needed with no records kept?
6. Who controls scrap? Is it properly segregated, stored, and kept free of dirt and trash?

4.13: Other casting materials; fusible alloys and plastics

There are many other materials besides casting alloys that can be used to cast rubber molds by the CRMC process. These materials make it possible to use the CRMC process in a wide variety of industrial and production applications.

Fusible alloys: Fusible alloys are low temperature alloys used primarily in industrial applications such as safety fuses designed to melt at a specific temperature to activate fire sprinkler heads and fire alarms, work holding devices, and solders. Most are eutectic alloys with temperatures that range from 117°F to 338°F.

Fusible alloys have been cast successfully in CRMC, but because of their low melting points, they require a longer spin cycle in order to solidify. As the quality of the alloy is critical, there can be no compromise in their selection. The reputation of the alloyer is the primary guide. A major supplier of these alloys is the Arconium Corporation of America, a member of the Oster Group.

Solder preforms: Because most solders can be cast, preforms can easily be made in CRMC. The process is important for producing solder preforms of unusual configurations that are not available by any other method. A major supplier of both solders and fluxes is Fry Metals Inc., a member company of the Oster Group.

Zinc aluminum: Because zinc aluminum is a higher temperature alloy than the tin/lead alloys, it is best to cast Zn/Al using silicone rubber molds. They may also be cast in organic rubber compounds, but mold life is considerably shortened because of the higher heat, and because the zinc reacts chemically with organic rubber. Gating and venting are essentially the same as for tin/ lead alloys.

Plastics: Casting plastics in rubber molds using the CRMC process is a versatile production method that opens up an entire new field of casting. Plastics should be cast in silicone rubber only because the silicone compounds re- lease plastic more easily and do not react chemically with the plastics. The two plastics most often used in CRMC are:

Thermoplastics: Thermoplastics are compounds that melt and are cast at 365°F. They solidfy in the normal casting cycle in approximately 5 minutes. They are available in clear or colors. Thermoplastics are more economical to use than polyesters as the gates and defective castings can be remelted in the pot and reused. They do require a larger funnel and gating system.

Polyesters: Polyester is supplied as a clear liquid to which color is added. A hardener, 1% by volume, is added to the liquid at room temperature prior to the casting cycle. Polyester has a pot life of approximately 1½ minutes and will cure in the mold with 1-1½ minutes. Because of its great fluidity. gates must be smaller than they are in molds being cast with metal. Venting is usually not required.

4.14: A note on the re-use of gates

The ratio of 50% gates to 50% new alloy has been used in this book for the return of gates and rejected castings to the melting pot, only because it has become an accepted industry practice. However, based upon intensive research at the Tin Research Institute, Columbus, Ohio, any re-use of gates and rejected castings in the casting process has been proven to be a detriment to any casting alloy.

One of the important functions of any gating system is to provide only clean alloy to the cavities in the mold by the entrapment of dirt and dross. An inspection of any cast gates (especially the basin area) will confirm that dirt, dross and porosity have been prevented from entering the castings. When gates are re-melted into an alloy, these undesirable elements are re-introduced thus defeating the purpose of having only clean, virgin alloys for casting. The use of only virgin alloys in casting has been proven by all the major jewelry manufacturing companies in the industry. The basis of both their quality and reputations are based upon this simple theory. The proof of this can be shown in any casting operation by simply comparing the results of using only new alloy vs. using an alloy in which gates have been remelted.

Gates are a viable commodity and as such retain a high proportion of their value. The Oster Company does not use this material in the manufacture of virgin Ostercast Alloys, but will accept them for the exclusive use of their sister company, Fry’s Metals Company (Formerly National Lead Industries). Fry’s redefines this material in their refinery into much less noble alloys strictly for use in the manufacture of body solders for the automotive field and as babbit metal and wheel weights for the industrial trade.

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